The present invention relates to novel photohardenable compositions and photosensitive materials employing the same. More particularly, it relates to free radical addition polymerizable compositions containing a photoreducible dye and a thiol as the photoinitiator.
U.S. Pat. No. 4,399,209 and 4,440,846 to The Mead Corporation describe imaging materials and processes in which images are formed through exposure controlled release of an image-forming agent from a microcapsule containing a photohardenable composition. The imaging material is image-wise exposed to actinic radiation and subjected to a uniform rupturing force such as pressure. Typically, the image-forming agent is a color precursor which is released from the microcapsule and reacts with a developer to form a visible image.
One of the problems which has been encountered in designing commercially acceptable full color imaging materials has been the relatively short wavelength band to which most photohardenable compositions are sensitive. In most cases, compositions are sensitive to ultraviolet radiation or blue light. Compositions which are sensitive to green or red light are few in number.
Full color photosensitive materials employing photohardenable microcapsules are described in U.S. application Ser. No. 339,917 and U.S. Pat. No. 4,576,891. These materials include a photosensitive layer containing three sets of photohardenable microcapsules. Each set of microcapsules is sensitive to a different band of radiation and contains a cyan, magenta or yellow image-forming agent. The absorption spectra of the initiators employed in these microcapsules are never perfectly distinct. There is always some degree of overlap in the absorption curves and, as a result, exposure conditions must be carefully controlled to avoid cross-exposure.
It would at least be desirable to extend the sensitivity of photohardenable compositions to longer wavelengths and it would be particularly desirable to extend the sensitivity of photohardenable compositions throughout the visible spectrum. By extending sensitivity deeper into the visible spectrum, the amount of overlap in the absorption spectra of the initiators can be reduced. When sensitivity is extended throughout the visible spectrum, a panchromatic system results.
Dye-sensitized photopolymerizable compositions are well known in the art. An excellent summary of them is provided by Eaton, "Dye Sensitized Photopolymerization", Advances in Photochem, Vol. 13, pgs. 427-487. Typically, dye-sensitized photopolymerizable compositions include a photoreducible or photoxidizable dye and a reducing agent or an oxidizing agent for the dye. An early example of a dye-sensitized photopolymerizable composition is provided by Oster in U.S. Pat. No. 2,875,047 which teaches a composition comprising vinyl compounds, a photoreducible dye and a reducing agent. Typical dyes include Rosebengal, Rhodamine B, Brilliant Green, and thionine. Typical reducing agents are stannous chloride, ascorbic acid, glutathione, thiourea, allyl thiourea and hydrazine. Other examples of dye sensitized compositions are provided in U.S. Pat. Nos. 3,488,269 to Allen et al.; 3,573,922 to Rust; 3,531,281 to Rust; and 3,650,927 to Livinos disclosing a combination of a photoreducible dye and a sulfone.